Method of spin-on-glass etchback using hot backside helium

ABSTRACT

A workpiece with a back surface and a front surface has a layer formed on the front surface thereof which is to be etched by plasma etching. The workpiece is placed on a lower electrode in a plasma etching system with the back surface resting on the lower electrode. The workpiece is clamped to the lower electrode. A gas circulation system is formed in the surface of the lower electrode to supply heated gas, under pressure, to the back surface of a workpiece placed thereon to cause the workpiece to bow thereby forming a vaulted space below the workpiece. Then, while heating the back of the workpiece in this way, plasma etching of the layer upon the workpiece is performed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to apparatus and systems for forming aSpin-On-Glass (SOG) layer on a silicon substrate and more particularlyto apparatus and a method for performing such a process in an etch backprocess.

2. Description of Related Art

In the past Spin-On-Glass has been etched back in a conventional plasmaetching system. There is a need for improvement in the methods and theapparatus used to perform the method to secure improved products and tomaximize the lifetime of the manufacturing tools.

SUMMARY OF THE INVENTION

In accordance with this invention, a method is provided for plasmaetching a layer of a material formed upon a substrate comprising thefollowing steps. Place a workpiece having a back surface and a frontsurface with the front surface having a layer formed thereon which is tobe etched by plasma etching on a lower electrode in a plasma etchingsystem. Clamp the workpiece upon the lower electrode, which has a gascirculation system formed in the surface thereof for supplying gases tothe lower surface of the workpiece. Supply a heated gas through the gascirculation system under pressure to the back surface of the workpieceto bow the workpiece forming a vaulted space therebelow. Plasma etch thelayer upon the workpiece.

Preferably, the gas employed comprises helium; the gas is heated to atemperature between about 70° C. and about 110° C. The gas is heated bya heater; the heater is controlled by a temperature controller; thepressure above the workpiece is between about 200 milliTorr and about600 milliTorr and the pressure in the vaulted space below the workpieceis between about 8 Torr and about 14 Torr.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects and advantages of this invention areexplained and described below with reference to the accompanyingdrawings, in which:

FIG. 1 shows an etch back process for a silicon wafer which has beenpreviously coated with a SOG (Spin-On-Glass) layer on the upper surfaceof a wafer, using the etchback process is used to planarize the SOGlayer on the wafer.

FIG. 2 shows an etch back process for planarizing a layer ofSpin-On-Glass on a silicon wafer in accordance with this inventionemploying a backside helium cooling system for heating instead ofcooling in an improved process in accordance with this invention.

FIG. 3 shows a system for producing a plasma of the kind provided by thesystem of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a system for use in an EB (Etch Back) process for a siliconwafer 11 which has been previously coated with a SOG (Spin-On-Glass)layer 14 on the upper surface 19 of wafer 11. The EB process is used toplanarize the SOG layer 14 on the wafer 11. The process is performedwithout using a backside helium cooling system of the kind provided inconventional plasma etching systems. The processing chamber 10 includesa cylindrical clamp 12 between which a plasma 16 is generated above thesemiconductor wafer 11 with ions 17 being driven from the plasma 16 downonto the SOG layer 14 on the upper surface 19 of wafer 11.

The kinetic energy of the ions 17 and the heat of the chemical reactionoccurring in the plasma heats the SOG layer 14 and the wafer 11. In mostplasma etching processes, a cooling gas (such as helium which is inert)has been used to prevent the wafer 11 from overheating. However in thecase of the SOG process of this invention, it has been found that itrequired that the wafer remain at a relatively high temperature to avoidpolymer deposition created by carbon and hydrogen from amethylpolyfluoride CHF_(x) (polytetrafluoroethylene-like (Teflon-like)deposits) during the process. Accordingly no cooling with helium is usedduring the plasma process shown in FIG. 1 because helium cooling gas atroom temperature removes the heat from the wafer very fast, which wasfound to induce poor uniformity.

In FIG. 1, a system is shown in which the heat, obtained from EB (EtchBack) plasma process for etching the SOG layer 14, is transferred to thewafer, and then to the lower electrode directly by conduction withoutusing the room temperature helium as a cooling medium. Finally, the heatis removed by cooling water passing through passageways 26 in lowerelectrode 20.

The wafer 11 has a lower surface 19' which rests on the top surface 20'of a lower metal electrode 20 which conducts heat away from lowersurface 19' of the wafer 11 as indicated by the phantom lines 21representing flow of heat flux through the electrode 20. Theconventional lower electrode 20 in a chamber includes a forced flow gascooling system which is not used in the embodiment of FIG. 1.

The unused cooling system of FIG. 1 includes an inlet pipe 22 connectedto at least one inlet passageway hole 23, which would normally be usedfor passage of a cooling gas to cool the wafer 11 and one outletpassageway hole 24 through electrode 20 extending from the back of thewafer 11 through an outlet pipe 25 which would normally be used forexhaust of a cooling gas from cooling of the wafer 11. However, theforced flow gas cooling system is not used in the implementation of FIG.1 for the reason that the passage of cooling gases cools the wafer 11causing the unwanted CHF_(x) (polytetrafluoroethylene-like(Teflon-like)) polymer deposits described above. We have discovered thatit is necessary to keep the wafer 11 at a relatively high temperature ofat least 60° C. to the 120° C. in order to prevent the deposition of themethylpolyfluoride CHF_(x) polymers from the plasma onto the wafersurface.

However, in the embodiment of FIG. 1 the cooling technique contemplatedis that the wafer 11 contacts the lower electrode 20 directly and heatfrom the wafer, created by the plasma process is transferred to thelower electrode by conduction. A problem has been discovered in that byusing the implementation of FIG. 1 that polymer or particle depositionalso occurs on the lower electrode 20 and the result is an imperfectupper surface 20' of the lower electrode 20. The imperfect surface 20'comprises the original surface as well as the surface which is theresult of the polymer or particle deposition on that surface. Theresulting imperfect upper surface 20' results in non-uniform heatexchange, which induces a poor SOG (Spin-On-Glass) EB (Etch Back)uniformity, which also reduces the life time of the lower electrode 20.

Most etching processes need a cooling gas on the backside surface ofwafer to remove the heat created from a plasma process in order toprotect the wafer from overheating. The most widely used cooling gas inthe etching process is room temperature helium. However, the current SOG(Spin-On-Glass) process needs a relatively high temperature remaining onthe wafer to avoid polymer deposition created by methylpolyfluorideCHF_(x) during process, so there is no helium used during plasmaprocess, because at room temperature helium removes the heat from thewafer very fast, which induces a poor uniformity.

The heat obtained from the SOG (Spin-On-Glass) EB (Etch Back) plasmaprocess of FIG. 1 is transferred to the wafer 11, and then to the lowerelectrode 20 directly by conduction without the room temperature heliumas a medium. Finally the heat is removed by cooling water.

Due to the imperfect surface 20' of the lower electrode 20 and a littlepolymer or particle deposition on the lower electrode after certainprocess cycles, the contact surface between the wafer 11 and the lowerelectrode 20 is not uniform and smooth, so the heat exchange is notuniform. It affects the SOG (Spin-On-Glass) EB (Etch Back) uniformityand results in deviation from specifications. At the same time, the lifetime of lower electrode 20 is also reduced.

In accordance with this invention, a hot helium gas is used as a mediumto provides heat to a wafer and preserves the heat of the wafer duringthe plasma etching process. By injecting hot helium gas onto thebackside of the wafer, the high wafer temperature requirement isachieved, so the polymer or the particle deposition on the wafer isavoided. Moreover, by using the hot helium as a heat exchange medium,the heat exchange between the wafer and the lower electrode is moreuniform, so the SOG (Spin-On-Glass) uniformity is improved and itsdeviation from specifications is improved. The damage on the lowerelectrode is also improved, so the life time of the lower electrode isincreased.

FIG. 2 shows a EB (Etch Back) process for planarizing a layer of SOG(Spin-On-Glass) 34 on a silicon wafer 31 in accordance with thisinvention employing a backside helium cooling system for heating insteadof cooling in an improved process in accordance with this invention. Theprocessing chamber 30 includes clamp cylinder 32 (shown in section)between which a plasma 36 is generated above a semiconductor wafer 31with ions 37 being driven from the plasma down onto the SOG layer 34 onthe upper surface 39 of wafer 31. The clamp 32 is a ceramic cylinder.The clamp 32 is secured to an upper chamber element, such as element 80of the system shown in FIG. 3.

FIG. 3 shows a system for producing a plasma of the kind provided by thesystem of FIG. 2. A disc-shaped semiconductor wafer 92 is supported onsupport 91 which is carried on lower chamber element 82. An annularclamp 86 is shown with a circular opening 86' therethrough. Opening 86'is aligned with the wafer 92 therebelow. The clamp 86 is supported by aplurality of suspension elements 84 comprising plungers with springssuspended at the upper ends thereof from upper chamber element 80. Achamber 90 is provided. The upper chamber element 80 is raised up torelease the clamp 86 and is lowered down to clamp the wafer 92. There isa match box (with an impedance matching network) and split PCB (printedcircuit board) element 93 at the right end of chamber 90 which ispowered by RF generator 96 which is connected to PCB element by line 94.The process uses RF power between about 300 watts and about 600 watts.The process uses pressure between about 200 mTorr and about 600 mTorr.

Applicant has discovered that despite the fact that the kinetic energyof the ions 37 and the heat of the chemical reaction occurring in theplasma heats the wafer 31, in contradiction to conventional practices inthe state of the art, that it is unnecessary that such heat bedissipated by either cooling gases or direct thermal contact with theelectrode 40 to avoid overheating of wafer 31 caused by the plasma. Ascan be seen, in the exaggerated drawing of FIG. 2, a vaulted or archedspace 35 has been formed because the workpiece 31, 34 (wafer 31/SOGlayer 34) has been temporarily bowed (i.e. or warped to form a cuppedconcave lower surface 39' and a convex or arched upper surface 39) by apressure gradient caused by the greater pressure on the lower surface39' of from about 8 Torr to about 14 Torr as compared to the lowpressure on the upper surface 39 of from about 200 milliTorr to about600 milliTorr (depending on process recipes) during the process ofheating with the helium gas in arched space 35. The pressure above theworkpiece 31, 34 is between about 200 milliTorr and about 600 milliTorr(depending on process recipes). The pressure in the vaulted space belowthe workpiece 31, 34 is between about 8 Torr and about 14 Torr(depending on process recipes).

The wafer is clamped into position on the periphery so that the pressuredifferential is maintained by the walls of clamp 32. The clampingemployed is illustrated by FIG. 3.

Since the upper surface 39 is convex and the lower surface 39' isconcave only the periphery of the lower surface 39' rests on the lowermetal electrode 40 of the plasma etching chamber 30. Instead ofconductive contact with wafer 31, thermally conductive metal electrode40 conducts some heat away from the hot gases in a thermal convectiontype cooling process within the vaulted or arched space 35 between thelower surface 39' of the workpiece 31, 34. Heat conducted through theelectrode 40 is indicated by the phantom lines 41 representing flow ofheat flux through the electrode 40. The conventional lower electrode 40in the chamber includes the forced flow gas cooling system which is usedin the embodiment of FIG. 2 which in this case is employed as a forcedflow gas heating system.

In accordance with the embodiment of FIG. 2, hot helium gas is used as amedium to provide heat to the back surface 39' of the workpiece 31, 34and preserving the heat of workpiece 31, 34 during plasma process. Byinjecting the hot backside helium, the high temperature requirement onthe wafer is achieved, so the polymer or the particle deposition on thewafer can be avoided. Moreover, by using the hot helium as a heatexchange medium, the heat exchange between the wafer and the lowerelectrode are more uniform and overall, so the SOG (Spin-On-Glass)uniformity and its deviation are improved. The damage on the lowerelectrode 40 is also improved, so the lifetime of the lower electrode 40is increased.

The "cooling system" used for distributing heating gas in FIG. 2includes an inlet pipe 42 connected to at least one inlet passagewayhole 43 through electrode 40 to outlet 43', which would normally be usedfor passage of a cooling gas to cool the workpiece 31, 34 and one outletpassageway hole 44 extending via opening 44' through electrode 40 fromthe vaulted lower surface 39' of the workpiece 31, 34 to an outlet pipe45 through which heating gas is exhausted after heating the vaultedspace below the workpiece 31, 34.

As in FIG. 1, it is necessary to keep the workpiece 31, 34 at arelatively high temperature of at least from 60° C. to about 120° C. inorder to prevent the deposition of methylpolyfluoride CHF_(x) polymersfrom the plasma onto the wafer surface. The temperature of the SOG layer34 and the top surface 39 the workpiece 31, 34 is to be between about80° C. and about 120° C. The temperature of bottom surface 39' of theworkpiece 31, 34 is to be between about 70° C. and about 110° C.

The helium gas 47 being admitted into line 42 is heated by a heater 48containing a helical electrical heating wire 48' wrapped about line 42to heat the incoming helium gas 47. Thermocouple unit 50 fastened tooutlet pipe 45 measures the temperature of gas from the vaulted chamber35. Thermocouple unit 50 is connected via line 51 to temperaturecontroller 52 which is connected via line 53 to control supply of powerto heater 48.

The temperature of the gases 47 at the inlet to passageway 42 from pipe42 is between about 20° C. and about 25° C.

The result is more uniform heat exchange, good, uniformity afteretchback (EB) of spin-on-glass (SOG) layers and reduced deviation fromspecifications, less deposition of polymers and particles and a longerlifetime for the lower electrode 39.

Finally the heat is removed by cooling water passing through passageways46 in lower electrode 40.

SUMMARY

The hot backside helium works as a medium of uniform heat exchangebetween the workpiece 31, 34 and the vaulted space 35 above the lowerelectrode 40.

The hot backside helium instead of room temperature helium (which is notused to remove the heat of wafer) is used to provide heat to theworkpiece 31, 34 and to preserve the heat of workpiece 31, 34 at acertain level that is critical for uniformity during the etchback (EB)process for an SOG layer 34. The hot backside helium provides a bettercontrol for the deviation of the EB (Etch Back) uniformity SOG(Spin-On-Glass) layer 34. The hot backside helium protects the lowerelectrode 40 from the damage of the etching process.

The improvements obtained by this invention are improvement of the SOG(Spin-On-Glass) uniformity and its deviation; reducing the polymer andparticle deposition on wafer, and increasing the life time of lowerelectrode.

While this invention has been described in terms of the above specificembodiment(s), those skilled in the art will recognize that theinvention can be practiced with modifications within the spirit andscope of the appended claims, i.e. that changes can be made in form anddetail, without departing from the spirit and scope of the invention.Accordingly all such changes come within the purview of the presentinvention and the invention encompasses the subject matter of the claimswhich follow.

Having thus described the invention, what is claimed as new anddesirable to be secured by Letters Patent is as follows:
 1. A method ofplasma etching a layer of a material formed upon a substrate comprisingthe steps as follows:placing a workpiece on a lower electrode in aplasma etching system, said workpiece having a back surface and a frontsurface, said back surface resting on said lower electrode, said frontsurface having a layer formed thereon which is to be etched by plasmaetching, clamping said workpiece upon said lower electrode, said lowerelectrode having a gas circulation system formed in the surface thereoffor supplying a heated gas to said back surface of said workpiece,supplying said heated gas through said gas circulation system underpressure to said back surface of said workpiece to bow said workpieceforming a vaulted space below said back surface, and plasma etching saidlayer upon said workpiece.
 2. A method in accordance with claim 1wherein:said heated gas supplied under pressure to said back surface ofsaid workpiece comprises helium gas.
 3. A method in accordance withclaim 1 wherein:said gas supplied under pressure to said back surface ofsaid workpiece is heated to a temperature between about 70° C. and about110° C.
 4. A method in accordance with claim 1 wherein:said gas suppliedunder pressure to said back surface of said workpiece comprises helium,and said helium gas is heated to a temperature between about 70° C. andabout 110° C.
 5. A method in accordance with claim 1 wherein:said gassupplied under pressure to said back surface of said workpiece is heatedby a heater.
 6. A method in accordance with claim 1 wherein:said gassupplied under pressure to said back surface of said workpiece is heatedby a heater, and said heater is controlled by a temperature controller.7. A method in accordance with claim 1 wherein:said heated gas suppliedunder pressure to said back surface of said workpiece comprises heliumgas, and a heater controlled by a temperature controller heats said gas.8. A method in accordance with claim 1 wherein:said gas supplied underpressure to said back surface of said workpiece comprises helium, saidhelium gas is heated to a temperature between about 70° C. and about110° C., and a heater controlled by a temperature controller heats saidgas.
 9. A method in accordance with claim 1 wherein:said pressure abovesaid workpiece is between about 200 milliTorr and about 600 milliTorr,and said pressure in said vaulted space below said workpiece is betweenabout 8 Torr and about 14 Torr.
 10. A method in accordance with claim 1wherein:said heated gas supplied under pressure to said back surface ofsaid workpiece comprises helium gas, said pressure above said workpieceis between about 200 milliTorr and about 600 milliTorr, and saidpressure in said vaulted space below said workpiece is between about 8Torr and about 14 Torr.
 11. A method in accordance with claim 1wherein:said gas supplied under pressure to said back surface of saidworkpiece is heated to a temperature between about 70° C. and about 110°C., said pressure above said workpiece is between about 200 milliTorrand about 600 milliTorr and said pressure in said vaulted space belowsaid workpiece is between about 8 Torr and about 14 Torr.
 12. A methodin accordance with claim 1 wherein:said gas supplied under pressure tosaid back surface of said workpiece comprises helium, and said heliumgas is heated to a temperature between about 70° C. and about 110° C.,said pressure above said workpiece is between about 200 milliTorr andabout 600 milliTorr, and said pressure in said vaulted space below saidworkpiece is between about 8 Torr and about 14 Torr.
 13. A method inaccordance with claim 1 wherein:said gas supplied under pressure to saidback surface of said workpiece is heated by a heater, said pressureabove said workpiece is between about 200 milliTorr and about 600milliTorr, and said pressure in said vaulted space below said workpieceis between about 8 Torr and about 14 Torr.
 14. A method in accordancewith claim 1 wherein:said gas supplied under pressure to said backsurface of said workpiece is heated by a heater, and said heater iscontrolled by a temperature controller, said pressure above saidworkpiece is between about 200 milliTorr and about 600 milliTorr andsaid pressure in said vaulted space below said workpiece is betweenabout 8 Torr and about 14 Torr.
 15. A method in accordance with claim 1wherein:said heated gas supplied under pressure to said back surface ofsaid workpiece comprises helium gas, and a heater controlled by atemperature controller heats said gas, said pressure above saidworkpiece is between about 200 milliTorr and about 600 milliTorr andsaid pressure in said vaulted space below said workpiece is betweenabout 8 Torr and about 14 Torr.
 16. A method in accordance with claim 1wherein:said gas supplied under pressure to said back surface of saidworkpiece comprises helium, said helium gas is heated to a temperaturebetween about 70° C. and about 110° C., and a heater controlled by atemperature controller heats said gas, said pressure above saidworkpiece is between about 200 milliTorr and about 600 milliTorr andsaid pressure in said vaulted space below said workpiece is betweenabout 8 Torr and about 14 Torr.
 17. A method of plasma etching a layerof a material formed upon a substrate comprising the steps asfollows:placing a workpiece on a lower electrode in a plasma etchingsystem, said workpiece having a back surface and a front surface, saidback surface resting on said lower electrode, said front surface havinga layer formed thereon which is to be etched by plasma etching, clampingsaid workpiece upon said lower electrode, said lower electrode having agas circulation system formed in the surface thereof for supplyingheated inert gas to said back surface of said workpiece, heating saidinert gas with a heater controlled by a temperature controller,supplying said heated inert gas through said gas circulation systemunder pressure to heat said back surface of said workpiece therebybowing said workpiece to form a vaulted space below said back surface,plasma etching said layer upon said workpiece, maintaining said pressureabove said workpiece between about 200 milliTorr and about 600 milliTorrand said pressure in said vaulted space below said workpiece is betweenabout 8 Torr and about 14 Torr.
 18. A method of plasma etching a layerof a material formed upon a substrate comprising the steps asfollows:placing a workpiece on a lower electrode in a plasma etchingsystem, said workpiece having a back surface and a front surface, saidback surface resting on said lower electrode, said front surface havinga layer formed thereon which is to be etched by plasma etching, clampingsaid workpiece upon said lower electrode, said lower electrode having agas circulation system formed in the surface thereof for supplyingheated helium gas to said back surface of said workpiece, heating saidgas with a heater controlled by a temperature controller to atemperature between about 70° C. and about 110° C. with a heatercontrolled by a temperature controller, supplying said heated helium gasthrough said gas circulation system under pressure to heat said backsurface of said workpiece thereby bowing said workpiece to form avaulted space below said back surface, plasma etching said layer uponsaid workpiece, maintaining said pressure above said workpiece betweenabout 200 milliTorr and about 600 milliTorr and maintaining saidpressure in said vaulted space below said workpiece between about 8 Torrand about 14 Torr.